Inkjet printers employing Memjet® technology are commercially available for a number of different printing formats, including home-and-office (“SOHO”) printers, label printers and wideformat printers. Memjet® printers typically comprise one or more stationary inkjet printheads, which are user-replaceable. For example, a SOHO printer or a benchtop label printer comprises a single user-replaceable multicolor (polychrome) printhead; a high-speed web printer comprises a plurality of user-replaceable monochrome printheads aligned along a media (web) feed direction (see, for example, US2012/0092403 and U.S. Pat. No. 8,398,231); and a wideformat printer comprises a plurality of user-replaceable multicolor printheads in a staggered overlapping arrangement so as to span across a wideformat pagewidth (see U.S. Pat. No. 8,388,093).
Inkjet nozzles must be maintained in a hydrated state in order to function properly. If a nozzle is not fully hydrated, the nozzle tends to become clogged with ink (“decapped”) and may be unable to eject a droplet of ink in response to a fire signal. Even if a dehydrated nozzle is still able to eject ink in response to a fire signal, the ejected droplet may be misdirected, have a reduced droplet volume or a reduced ejection velocity if not fully hydrated, any of which may lead to a reduction in print quality. The problem of nozzle dehydration is particularly exacerbated in Memjet® printers, which generally have low droplet volumes (e.g. 1-3 pL) and dendritic ink supply channels.
Inkjet printers usually employ various strategies for unclogging nozzles or restoring nozzles to a fully hydrated state. Typically, this involves a maintenance cycle which may comprise wiping, forced ink purging (e.g. by a applying a vacuum to the nozzle plate or a positive pressure to the ink supply) and firing ink droplets into a spittoon (“spitting”). Spitting may involve increasing the usual droplet ejection energy to force ink from nozzles (see, for example, US 2011/0310149, the contents of which are incorporated herein by reference). Spitting may be performed during a maintenance cycle or between media sheets during a print job.
Inkjet printers may additionally employ various strategies for maintaining nozzles in a hydrated state and, thereby minimizing the frequency of maintenance interventions required. Maintenance interventions for restoring nozzles to a functioning state are time-consuming and wasteful of ink and should be avoided as far as possible. Maintenance inventions are potentially problematic when printing onto a media web, because a conventional maintenance station cannot cross the media path without cutting the web. Moreover, between-page spitting is not an option when printing onto a continuous media web.
One strategy for minimizing clogging of non-firing nozzles uses sub-ejection pulses which have insufficient energy to eject a droplet of ink, but sufficient energy to warm the ink inside the nozzle chamber and thereby reduce its viscosity. The use of sub-ejection pulses in this manner is described in U.S. Pat. No. 7,845,747, the contents of which are incorporated herein by reference.
Another strategy for minimizing clogging of nozzles is to ensure that each nozzle of the printhead is fired periodically so that the ink inside the nozzle chamber is continuously refreshed and does not have an opportunity to dehydrate. U.S. Pat. No. 7,246,876, the contents of which are incorporated herein by reference, describes printing a low-density keep-wet pattern onto a media substrate to ensure that each nozzle of the printhead is fired within a time period which is less than a decap time of the nozzle. Typically, the density of dots on the media substrate by virtue of the keep-wet pattern is less than 1:250 and not clustered so as to minimize visibility.
Keep-wet patterns are potentially an important strategy for maintaining good print quality in inkjet printers, especially inkjet web printers, where this no opportunity for between-page spitting and less opportunity for maintenance interventions. However, keep-wet patterns paradoxically reduce print quality by printing additional dots, which are not part of the image data sent to the printer. It would therefore be desirable to minimize the visibility of keep-wet patterns and further improve print quality, especially in inkjet web printers which cannot perform between-page spitting.